1. Field of the Invention
This invention relates to an image forming device such as electrophotographic device or electrostatic recording device, etc., typically a copying machine, laser beam printer, etc., and particularly to an image forming device equipped with a transfer means having a moving member which moves while pressing a recording material against an image carrying or bearing member as the means for transferring the image on the image carrying member onto the recording material, namely the so called contact type transfer means.
2. Related Background Art
The image forming device of the prior art by use of the transfer means of the contact type is described by referring to a laser beam printer as shown in FIG. 1.
The unfixed toner image formed on the photosensitive drum by the primary charger 8, the laser beam scanner 1, and the developing device 3 each arranged respectively around the photosensitive drum 2 as the image carrying member, is transferred by the transfer roller 4 pressure contacted with the photosensitive drum 2 onto the recording material 5. The residual toner after transfer is recovered from the photosensitive drum 2 by the cleaning device 7. On the other hand, the toner image transferred onto the recording material is fixed by the fixing device 14.
FIG. 2 is an enlarged sectional view of the fixing device 14.
The fixing device 14 has a pair of rollers 15, 16 arranged in pressure contact with each other, and fixing is carried out by heating the rollers by means of a heating means. The fixing roller 15 is constructed of a fluorine resin layer provided on the outer surface of a hollow aluminum core metal, and a halogen heater 11 is arranged therein. The member 17 functions as both a silicone oil coating device and a cleaning device, and comprises the cleaning member 17A made of a heat-resistant felt impregnated with a silicone oil. A holder 17B is supporting cleaning member 17A, which is pressed against the fixing roller 15. The pressurizing roller 16 has the elastic layer 16A made, for example, of the low temperature vulcanization type silicone rubber (LTV) around the core metal 16B.
The contact transfer device as represented by the transfer roller 4, which can perform transfer at lower voltage as compared with corona transfer device and therefore can transfer and convey a diversity of recording materials from thin paper to envelope, having further the advantage of little generation of ozone, and further excellent in conveying force of the recording material. As still another advantage, there is little poor or fault image generated during transfer. For this reason, it is effective for a compact electrophotographic printer, etc.
However, in the image forming device having the transfer roller 4 and the heating roller fixing device 14 of the prior art as described above, the following problems have been found.
(1) The toner on the image carrying member, the paper powder generated from the recording material, etc. are conveyed from the transfer roller 4 onto the back surface of the recording material and then onto the pressurizing roller 16, whereby mold releasability of the pressurizing roller is lowered. For this reason, the pressurizing roller 16 in the heating roller fixing device 14 is contaminated, and as the progress of contamination, such phenomena as luster irregularity of the fixed image, back contamination of the recording material, winding of the recording material around the pressurizing roller, etc. are generated.
Accordingly, the life of the pressurizing roller becomes shorter, and as compared with durability of about 100,000 sheets when using a corona transfer device, durability life is shortened to about 30,000 sheets on account of pressurizing roller contamination.
(2) The transfer roller 4 forms a roller pair with the photosensitive drum 2 which is an image carrying member and conveys the recording material. At this time, since their circumferential speed is not exactly the same as that of the roller pair within the fixing device, a stress is applied on the recording material between the transfer roller 4 and the fixing device 14. Due to the influence of the stress, wrinkle of the recording material is liable to be generated when passing through the fixing device. The phenomenon is more likely to occur with thin paper or envelope.
Whereas, concerning the above phenomenon (1), it can be solved by making cleaning of the transfer roller 4 complete, but provision of a cleaning member on the transfer roller 4 will make the device more complicated, and also poses a problem in disposal of the toner that has been cleaned off the transfer roller 4.
Further, for the transfer roller 4, an elastic material is employed for forming a nip between it and the photosensitive drum 2. At this time, if the nip is not formed under a low pressure, the so called void or hollow character phenomenon will be generated, which is the phenomenon in which the central portion of the character is not transferred. Therefore, a rubber of low hardness or a foamed sponge, etc. is used for the transfer roller 4. For this reason, the surface roughness of the transfer roller 4 is rough and also its coefficient of friction is high, thus also posing a problem that the toner can be cleaned from it only with difficulty.
On the other hand, concerning the phenomenon (2), wrinkles can be improved by making lower the pressing force of the transfer roller 4 against the photosensitive drum 2 and rotating said roller subject to the photosensitive roller. However, the shock from when the recording material enters into the fixing instrument or the shock when it has passed out of the feeding paper, the resist roller, poor or fault image is generated. Concerning this problem, it can be also solved by taking good distance between transfer and fixing to form a loop of the recording material, but it is difficult to take such space in a compact printer, etc.
Thus, in an image forming device by use of the contact type transfer device such as transfer roller as the transfer means, it has been difficult to effect fixing with excellent conveyability, durability.